When the OD light comes on and flashes there is a problem in the transmission. It could be something simple like a bad selenoid or major transmission problem. Take it to a transmission shop that has a good reputation, and have the transmission codes read. This will telln you what cost you are looking at.
The diagnostic trouble codes go a long way to help the technician diagnosing a transmission concern, but keep in mind, if the technician removed the transmission pan and found heavy metal debris from some of the many moving components, a replacement or over-haul is still necessary. A failed $3.00 washer in a transmission could come apart and take out thousands of dollars of components. I recommend asking questions as to why the shop come to the conclusion and what other options are available.